1. Field of the Invention
A nuclear reactor is a device that artificially controls the chain reaction of the nuclear fission of fissile materials, thereby achieving a variety of purposes such as the generation of heat, the production of radioisotopes and plutonium, the formation of radiation fields, or the like.
2. Background of the Related Art
A nuclear reactor is a device that artificially controls the chain reaction of the nuclear fission of fissile materials, thereby achieving a variety of use purposes such as the generation of heat, the production of radioisotopes and plutonium, the formation of radiation fields, or the like.
Generally, a nuclear fuel assembly basically includes a frame body comprised of a top nozzle, a bottom nozzle, a plurality of spacer grids, guide thimbles and an instrumentation tube, and a plurality of fuel rods held longitudinally in an organized array by the spacer grids spaced along the length thereof in such a manner as to be supported by means of springs and dimples disposed within the spacer grids. So as to prevent the formation of the scratches on the fuel rods and the generation of the damage on the springs within the spacer grids upon assembling the nuclear fuel assembly, thereafter, the fuel rods have a locker applied thereon and are then inserted into the frame body of the nuclear fuel assembly. Next, the top and bottom nozzles are secured to the opposite ends of the nuclear fuel assembly, thereby finishing the assembling procedure of the nuclear fuel assembly. Then, after the locker of the finished assembly is removed, the distances between the fuel rods, the distances between the fuel rods, the distortion of the nuclear fuel assembly, the total length thereof, and the dimension thereof are checked out, thereby finishing the manufacturing procedure of the nuclear fuel assembly.
FIG. 1 is a sectional view showing a general structure of a fuel rod.
Referring to FIG. 1, the fuel rod basically includes a plurality of fuel pellets 3 for generating heat in a nuclear fuel assembly; a cladding tube 1 for transmitting the heat to reactor core coolant and serving as a receptacle for the fuel pellets 3; a coil type compression spring 4 adapted to compress the fuel pellets 3 with given load for preventing the movements of the fuel pellets 3 in a length direction of the fuel rod, thereby preventing the damage and breakage of the fuel pellets 3 while the nuclear fuel assembly is transported and handled; and an upper end plug 5 and a lower end plug 2 adapted to be welded to the cladding tube 1 at the upper and lower ends of the cladding tube 1 for preventing the fuel pellets 3 inserted into the cladding tube 1 from escaping from the fuel rod after sealing (wherein the structure of the coil type compression spring is shown in more detail in FIG. 2).
In the assembling process of the fuel rod in the nuclear fuel assembly, the cladding tube 1 and the lower end plug 2 are first welded to each other, and after the fuel pellets 3 are inserted into the cladding tube 1, the coil type compression spring 4 is compressed against the fuel pellets 3 and inserted into the cladding tube 1 by means of the upper end plug 5. Next, the upper end plug 5 and the cladding tube 1 are welded to each other at an appropriate inert gas atmosphere.
As shown in FIG. 3, if the coil type compression spring 4 is compressed by means of the upper end plug 5 during the manufacturing of the fuel rod, buckling where the coil type compression spring 4 is bent may undesirably occur. Unfortunately, in case where the buckling of the coil type compression spring 4 occurs, the productivity of the fuel rod is remarkably reduced, and also, the upper end plug 5 that is adapted to completely separate the fuel pellets 3 from the reactor coolant is not welded well to the cladding tube 1.
The fuel pellets 3 are materials for producing the energy within the nuclear reactor, and in a light water nuclear reactor, enriched uranium obtained by raising a ratio of uranium-235 to a range between 2% and 5% is generally used. The enriched uranium is molded to a cylindrical pellet having a weight of about 5.2 g and a diameter of about 8.05 mm, such that about 356 fuel pellets are inserted into each fuel rod and about 96,000 fuel pellets are into each nuclear fuel assembly.
Through the nuclear fission of the fuel pellets 3, heat is generated from the nuclear fuel assembly, and during the nuclear fission, a great lot of gases are generated from the fuel pellets 3. The gases are accumulated in the interior of the fuel rod made by welding the cladding tube 1 to the upper end plug 5 and the lower end plug 2 during the burn-up of the nuclear fuel assembly, so that the internal pressure of the cladding tube 1 of the fuel rod is increased to generate the stress in the cladding tube 1 of the fuel rod, thereby damaging the integrity of the fuel rod.
So as to solve the above-mentioned problems, thus, there is proposed prior art U.S. Pat. No. 4,460,540 entitled ‘Burnable poison rod for a nuclear reactor fuel assembly’.
The prior art does not employ the coil type compression spring, but employs a clip type compression spring, which is shown in FIGS. 4a and 4b. 
FIG. 4a shows the clip type compression spring 10 prior to the insertion into the cladding tube 12. The clip type compression spring 10 is formed of a hollow circular tube and has a somewhat larger diameter than that of the cladding tube 12. Also, the clip type compression spring 10 is cut up and down along the side periphery thereof by a predetermined width, and when viewed at the top, it has a ‘C’-like shape.
Referring to FIG. 4b, since the diameter of the clip type compression spring 10 is larger than that of the cladding tube 12 when the clip type compression spring 10 is inserted into the cladding tube 12, the contraction of the clip type compression spring 10 in the hollow direction thereof occurs by the incised portion of the clip type compression spring 10 formed at the opposite side with respect to an elastic center point 11 formed by the shape characteristics of the clip type compression spring 10, thereby inserting the clip type compression spring 10 into the cladding tube 12. After the insertion, the clip type compression spring 10 applies a pushing force to the internal peripheral wall of the cladding tube 12 by the elastic restoring force thereof.
Since the clip type compression spring 10 has a larger outer diameter than the inner diameter of the cladding tube 12, however, the outer periphery of the clip type compression spring 10 is not completely brought into contact with the inner peripheral wall of the cladding tube 12 according to the difference between the outer diameter of the clip type compression spring 10 and the inner diameter of the cladding tube 12 and the material properties, when the clip type compression spring 10 comes into contact with the inner peripheral wall of the cladding tube 12, such that the assembling stability may be undesirably diminished.
As shown in FIG. 5, the clip type compression spring 10 makes the use of the elastic force in the hollow direction thereof, such that as the elastic force of the clip type compression spring 10 is decreased toward the elastic center point 11 thereof, the forces applied to the inner peripheral wall of the cladding tube 12 are different from one another, thereby generating irregular forces on the inner peripheral wall of the fuel rod. This causes excessive stress to be generated partially on the inner peripheral wall of the fuel rod, thereby damaging the integrity of the fuel rod.